Corrugated webs possess increased strength and dimensional stability compared to un-corrugated (i.e. flat) webs of the same material. For example, corrugated paperboard or cardboard is widely used in storage and shipping boxes and other packaging materials to impart strength. A typical corrugated cardboard structure known as ‘double-wall’ includes a corrugated paperboard web sandwiched between opposing un-corrugated paperboard webs referred to as ‘liners.’ The opposing liners are adhered to opposite surfaces of the corrugated web to produce a composite corrugated structure, typically by gluing each liner to the adjacent flute crests of the corrugated web. This structure is manufactured initially in planar composite boards, which can then be cut, folded, glued or otherwise formed into a desired configuration to produce a box or other form for packaging.
Corrugated webs such as paperboard are formed in a corrugating machine starting from flat webs. A conventional corrugating machine feeds the flat web through a nip between a pair of corrugating rollers rotating on axes that are perpendicular to the direction of travel of the web when viewed from above. Each of the corrugating rollers has a plurality of longitudinally-extending ribs defining alternating peaks and valleys distributed about the circumference and extending the length of the roller. The rollers are arranged so that their respective ribs interlock at the nip, with the ribs of one roller being received within the valleys of the adjacent roller. The interlocking ribs define a corrugating labyrinth through which the web travels as it traverses the nip. As the web is drawn through the corrugating labyrinth it is forced to conform to the configuration thereof, thus introducing into the web flutes or corrugations that approximate the dimensions of the pathway through the corrugating labyrinth. Accordingly, it will be appreciated that in a conventional corrugating machine flutes are introduced into the web along a direction that is transverse to the web-travel pathway; i.e. the flutes extend in a transverse (cross-machine) direction relative to the direction of travel of the web (machine direction). More simply, conventionally the flutes extend along the width of the web between its lateral edges. An example of this conventional methodology is shown in U.S. Pat. No. 8,057,621 (see FIGS. 7 and 7a thereof), which is incorporated herein by reference.
Corrugating a web in this manner can damage the paperboard or other web material because it introduces a substantial amount of oscillatory frictional and tension forces to the web leading into and while traversing the corrugating nip. Briefly, as the web is drawn between the corrugating rollers and forced to negotiate the corrugating labyrinth, the tension of the web, as well as compressive stresses normal to the plane of the entering web, oscillate in magnitude and direction as successive flutes are formed due to the reciprocating motion of the corrugating ribs relative to the web, and due to roll and draw variations in the web through the labyrinth as it is being corrugated. The oscillatory nature of the web tension through a corrugating labyrinth between corrugating rollers is well documented; see, e.g., Clyde H. Sprague, Development of a Cold Corrugating Process Final Report, The Institute of Paper Chemistry, Appleton, Wis., Section 2, p. 45, 1985. The resulting substantial cyclic peaks in web tension typically produce some structural damage in the web as it is corrugated.
In addition to undesirable tension effects, corrugating the web in the cross-machine direction introduces flutes that extend transverse to the fibers of the paperboard, which typically run the length of the web in the machine direction. Thus, flutes formed in a cross-machine direction must re-orient and introduce undulations into the paper fibers, which can also lead to reduced strength.
One way to address the aforementioned problems would be to corrugate the web in the machine direction so that the flutes extend along the direction of the web-travel pathway; i.e. in the longitudinal direction of the web itself. This is commonly referred to as ‘longitudinal corrugating’ or ‘linear corrugating.’ One issue with longitudinal corrugating is that as the longitudinally-extending flutes are formed, they necessarily consume web width (i.e. the extent of the web in the lateral, cross-machine direction) in order to convert the initially flat web into one having hills and valleys. In other words, to produce longitudinally-extending flutes the web must be gathered in the cross-machine direction such that its overall width after the flutes are formed is lower than the web width prior to forming the flutes. The ratio of the flat web's original, pre-corrugated width to its post-corrugated width is referred to as the ‘take-up ratio.’ Take-up ratios are well known for standard flute sizes in conventional transverse corrugating methods. For example, a conventional transversely-corrugated, A-fluted web exhibits a typical take-up ratio of 1.56 because the amplitude and pitch of A-flutes are such that introducing them into the web reduces the web length (i.e. its linear dimension in a direction transverse to the flutes) by 64%; i.e. making the ratio of starting length to ending length equal to 1.56. Stated another way, in conventional corrugating if one wants to end up with 100 yards of transversely-corrugated web, one has to feed 156 yards of flat web to the corrugating machine to account for the web length consumed by introducing the A-flutes.
A similar take-up ratio will be present in linear corrugating except that now that ratio will apply to the web's width in the cross-machine direction instead of to its length. This introduces a special problem because typical linear-corrugating devices such as linear-corrugating rollers cannot simultaneously gather web width and introduce corrugations without damaging and tearing the web. For example, linear corrugating rollers have circumferentially-extending ribs and valleys distributed longitudinally along the length of the rollers, wherein the circumferential ribs of one roller are received within the circumferential valleys of the opposing roller, and vice versa. Unless the web width is condensed sufficiently to account for the take-up ratio of the finished product prior to entering the nip between these rollers, it will be substantially wider than the intended product on entering the nip and would need to be instantaneously and simultaneously gathered and corrugated to produce the desired product. This cannot be achieved without damaging and tearing the web. To solve this problem, the traveling web should be gathered from its initial width to its approximate final width, based on the anticipated take-up ratio, prior to being introduced into the linear-corrugating rollers or other corrugating device.
For this reason, to date carrying out linear corrugating is impractical for commercial applications that require conventional flute sizes (e.g. A- through E-flutes) for useful web widths (e.g. final width of 50 inches). U.S. Pat. No. 7,691,045 (incorporated herein by reference) discloses a machine for gathering a traveling web laterally in the cross-machine direction prior to introducing that web to a set of rollers to introduce a three-dimensional pattern into the web. That machine utilizes a series of opposed rollers disposed along the machine direction for introducing longitudinal folds into the web beginning at the web's center. Each successive set of rollers thereafter introduces two additional folds at either side of the previously-made fold(s) until the entire web consists of a series of longitudinal folds or flutes so that the web's entire width has been gathered to a desired degree. This machine can be effective to gather the width of a paper or other web prior to downstream operations (such as corrugating or other three-dimensional forming) for relatively narrow widths that are not particularly useful on a commercial scale. Unfortunately, however, for commercial widths of, e.g. 50 inches or greater, the number of successive sets of opposed rollers that would be needed to successively form the longitudinal flutes is such that the machine would be impractically long, producing a very large footprint. Accordingly, such a machine is not capable of being retrofitted into existing corrugating lines where space is tight, and for new installations it would take up too much space to be practical.
U.S. Pat. Appl'n Pub. No. 2010/0331160 (incorporated herein by reference), which is commonly assigned with the present application, discloses another machine for gathering the width of a traveling web. That machine utilizes opposing sets of linear flute-forming bars that generally extend in the machine direction, wherein the spacing between adjacent ones of the bars generally decreases along the machine direction. The opposing sets of bars are interlaced such that the traveling web is caused to gradually conform to an intermediate longitudinally-fluted geometry as it passes between the opposing sets of bars by virtue of the decreasing lateral spacing between the bars. This machine has the advantage that it is capable of gathering the width of a traveling web in a relatively short distance of web travel, and is therefore of a practical size and footprint to be retrofitted into existing installations. However, as the paperboard web traverses the labyrinth between the opposing sets of flute-forming bars and is gathered laterally inward, individual paper elements in the web are dragged laterally across the bars thereby introducing position- and time-dependent lateral tension variations and oscillations throughout the web, which are undesirable and may contribute to damage.
It would be desirable to gather the width of a traveling web of material in the cross-machine direction according to a predetermined take-up ratio desirable for downstream processing, while minimizing or eliminating introduction of lateral tension or frictional forces in the web as a result of the gathering operation. The gathered web could then be introduced into downstream processing operations, such as longitudinal corrugating or other operations for introducing three-dimensional structure to the web, which downstream operation(s) will benefit from the lateral take-up ratio introduced in the earlier gathering operation.